The researchers also predict that, unless agricultural
policies change, the lake will continue to experience extreme blooms.
"The factors that led to this explosion of algal blooms
are all related to humans and our interaction with the environment," says
Bruce Hamilton, program director at the National Science Foundation (NSF),
which funded the research through its Water, Sustainability and Climate (WSC)
WSC is part of NSF's Science, Engineering and Education for
Sustainability (SEES) initiative.
Climate change partly
"Population growth, changes in agricultural practices
and climate change are all part of the equation," says Hamilton.
"These findings show us where we need to focus our attention in the
Results of the research are published in the journal Proceedings of the National Academy of Sciences
"The 'perfect storm' of weather events and agricultural
practices that occurred in 2011 is unfortunately consistent with ongoing
trends," says Anna Michalak, the paper's lead author and a scientist at
the Carnegie Institution for Science's Department of Global Ecology, located at
"That means that more huge algal blooms can be expected
in the future, unless a scientifically-guided management plan is implemented
for the region."
Freshwater algal blooms may result when high amounts of
phosphorus and nitrogen are added to the water, usually as runoff from
These excess nutrients encourage unusual growth of algae and
Drop in oxygen
When the plants and algae die, decomposers in the water that
feed on them use up oxygen, which can drop to levels too low for aquatic life
At first the Lake Erie algae were almost entirely
Microcystis, an organism that produces a liver toxin and can cause skin
The scientists combined sampling and satellite-based
observations of the lake with computer simulations to track the bloom.
It began in the lake's Western region in mid-July and
covered an area of 230 square miles.
At its peak in October, the bloom had expanded to more than
1 930 square miles. Its peak intensity was more than three times greater than
any other bloom on record.
The researchers looked at numerous factors that could have
contributed to the bloom, including land-use, agricultural practices, runoff,
wind, temperature, precipitation and circulation.
They found that three agriculture management practices in
the area can lead to increased nutrient runoff: autumn fertilization, broadcast
fertilization (uniform distribution of fertilizer over the whole cropped field),
and reduced tillage.
These practices have increased in the region over the last
Conditions in the fall of 2010 were ideal for harvesting and
preparing fields and increasing fertilizer application for spring planting.
Before the bloom
A series of strong storms the following spring caused large
amounts of phosphorus to flow into the lake.
In May alone rainfall was more than 6.5 inches, a level more
than 75% above the prior 20-year average for the month.
This onslaught resulted in one of the largest spring
phosphorus loads since 1975, when intensive monitoring began.
Lake Erie was not unusually calm and warm before the bloom.
But after the bloom began, warmer water and weaker currents encouraged a more
productive bloom than in prior years.
The longer period of weak circulation and warmer
temperatures helped incubate the bloom and allowed the Microcystis to remain
near the top of the water column. That had the added effect of preventing the
nutrients from being flushed out of the system.
The researchers' data did not support the idea that land-use
and crop choices contributed to the increase in nutrient run-off that fuelled
To determine the likelihood of future mega-blooms, the
scientists analyzed climate model simulations under both past and future
They found that severe storms become more likely in the
future, with a 50 percent increase in the frequency of precipitation events of
.80 inch or more of rain.
Stronger storms, with greater than 1.2 inch of rain, could
be twice as frequent.
The researchers believe that future calm conditions with
weak lake circulation after a bloom's onset are also likely to continue, since
current trends show decreasing wind speeds across the United States.
That would result in longer-lasting blooms and decreased
mixing in the water column.
"Although future strong storms may be part of the new
normal," says Michalak, "better management practices could be
implemented to provide some relief to the problem."
The research was also supported by the NOAA Center for
Sponsored Coastal Ocean Research and the Lake Erie Protection Fund.